You will be given a definition of a task first, then some input of the task.
In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D". 

[header] How to identify atrial fibrillation [title] Monitor any breathlessness you experience. [step] When your heart starts beating quickly, as it does when you have atrial fibrillation, the rest of your body gets less blood supply because your heart is beating less efficiently. When your body's blood supply slows down, oxygen levels in your blood decrease, which will make you feel like you can't catch your breath. <sep> (A) [substeps] Dilation is when the blood vessels in your heart contract. Stopping them helps blood flow more effectively and allows more blood to be drawn from the heart. (B) [substeps] Your heart isn't pumping as quickly as normal when you have atrial fibrillation. This can make it harder for the heart to work smoothly. (C) [substeps] You may experience this feeling of breathlessness when you are climbing the stairs, exercising, or simply just walking. [title] Notice if you feel like your heart is pounding. (D) But even if your heart stop beating rapidly, that still means your heart is not working efficiently or is working too fast. This is called atrial fibrillation.
Output:
C